In the related art, there has been developed a miniature antenna to be used for the communications of ultrashort waves. Especially in the communication standards called the UWB (Ultra-wideband), the communication rate can be raised, but the band to be used is usually as wide as 3.1 GHz to 10.6 GHz. Therefore, it has been desired to develop the antenna device, which can pick up electric waves of such wide range efficiently. In the related art, the biconical antenna or the discone antenna has been known as the antenna device having wideband frequency characteristics. In Japanese Patent No. 3,273,463, for example, there is disclosed a wideband antenna device using a semicircular radiation plate. With a view to reducing the size of the antenna device, moreover, there have been proposed antenna devices of various shapes to reduce the size of the wideband antenna such as a bow-tie antenna (JP-A-2002-135037).